The present invention relates to an On Die Thermal Sensor (ODTS), and a method for generating a thermal code of the ODTS, and more particularly, an ODTS and a method capable of increasing a temperature detecting region of the ODTS.
Generally, an ODTS is used for measuring a temperature in various semiconductor devices. Hereinafter, an application to a dynamic random access memory (DRAM), which is one of the semiconductor devices, is described.
A DRAM cell includes a transistor to be operated as a switch and a capacitor for storing a charge, i.e., data. According to whether the capacitor stores the charge, i.e., whether a terminal voltage of the capacitor is high or low, a logic level of the data is determined as a high level (logic 1) or a low level (logic 0).
Since the data is stored in the capacitor in the form of an accumulated electrical charge, it would be ideal that there is no power consumption of the stored charge. However, since there occurs a leakage current due to a PN junction of a metal oxide semiconductor (MOS) transistor, the stored initial charge may be discharged and, thus, the stored data may vanish.
To prevent data loss, the data stored in a memory cell is read and the read data is restored into the memory cell by recharging the memory cell with a normal charge before the data vanishes. This operation should be periodically performed in order to maintain stored data.
The above-mentioned recharging operation is called a refresh operation and, generally, a control of the refresh operation is performed by a DRAM controller. Due to the refresh operation, a refresh power is inevitably consumed. In a case of a battery operated system, which requires less power consumption, reducing power consumption for the refresh operation is very important and is a critical issue.
One method of reducing the power consumption for the refresh operation is changing a refresh period according to a temperature. As the temperature decreases, a data holding time of the DRAM becomes longer. Therefore, by dividing a temperature region into several temperature ranges and by lowering a frequency of a refresh clock at a lower temperature range, a power consumption can be reduced.
Accordingly, it has been required to develop a device for accurately sensing a temperature of the inside of the DRAM and for adjusting a refresh clock frequency.
FIG. 1 is a block diagram illustrating a conventional ODTS.
As shown, the conventional ODTS generally includes a band gap unit 10 and an analog-to-digital converting unit 20.
More specifically, the band gap unit 10 detects a temperature of the memory device based on the fact that a voltage level of a base-emitter voltage VBE of a bipolar junction transistor (BJT) changes according to a temperature variation in the ratio of about −1.8 mV/° C. By amplifying the base-emitter voltage VBE of the BJT which is changed finely, the band gap unit 10 outputs a first voltage VTEMP corresponding to the temperature. The base-emitter voltage VBE of the BJT becomes lower as the temperature becomes higher.
The analog-to-digital converting unit 20 converts the first voltage VTEMP output from the band gap unit 10 into a digital code DIGITAL_CODE to be outputted. Generally, a tracking analog-to-digital converter is used as the analog-to-digital converting unit 20.
The tracking analog-to-digital converter tracks the first voltage VTEMP by using a second voltage DACOUT to generate the digital code DIGITAL_CODE. Hereinafter, an operation of the tracking analog-to-digital converter is described in detail.
First, the tracking analog-to-digital converter compares the first voltage VTEMP with the second voltage DACOUT and increases or decreases the digital code DIGITAL_CODE according to the comparison result. At this time, the second voltage is increased or decreased along with the digital code DIGITAL_CODE and the increased or decreased second voltage is compared again to the first voltage VTEMP. By repeating the above process, the second voltage tracks the first voltage VTEMP, and the digital code DIGITAL_CODE corresponding to the first voltage VTEMP is outputted.
In short, when the band gap unit 10 outputs the first voltage VTEMP having temperature information, the analog-to-digital converting unit 20 converts the first voltage VTEMP into the digital code DIGITAL_CODE corresponding to the temperature information and outputs the digital code DIGITAL_CODE.
When the ODTS has a sensing ability for a broad temperature region, the power management and the system management of the memory device using the ODTS improves. This improvement may be obtained by a fine temperature management based on the detected temperature information of the broad range.
However, for the ODTS to cover the broad temperature region, there is a corresponding die dimension increase and a power consumption problem due to an increase in circuits for outputting additional information.